Cellulose is one of the most important biomass resources in the world. It is eco-friendly, biodegradable, sustainable and abundant. Cellulose is a very vital raw material in the industry and it has been broadly used in paper products, building materials, personal hygiene items, pharmaceuticals, and chemicals. However, it is rare to use cellulose as the major substrate in the electrical industry due to its nonconductive properties.
Cotton is an important natural resource for cellulose. As compared to other natural resources, cotton contains more than 90% cellulose which is much higher than wood cellulose (40%) while slightly lower than bacterial cellulose (92-98%) [1,2]. Cotton is also an important cash crop in the United States leading to the biggest cotton exportation in the world as well as a $25 billion-per-year industry in the country [3]. However, due to persistent increase of worldwide cotton production and substantial consumption of synthetic fibers, the market price of cotton as a raw material has been declining since 2009. Therefore, transforming cotton to value-added high-tech products is attracting tremendous interests.
The traditional application of cotton mostly serves the textile industry that rarely requires sophisticated processing to directly utilize raw cotton fibers. However, value-added or high-tech cotton derived products generally requires a pivotal step to effectively dissolve cotton cellulose. As compared to wood-derived cellulose, cotton-derived cellulose has a fairly high degree of polymerization (DP) of 9000-15000 with more than 66% crystallinity [4], which makes cotton-derived cellulose hard to dissolve in water and many organic solvents. The dissolution of cellulose is highly affected by its molecular weight and crystallinity [5]. Most researchers believe that the breakdown of the inter-chain hydrogen bonding of cellulose is the key to enhance its solubility [6]. Studies showed that solvents containing N-methylmorpholine-N-oxide (NMMO), ionic liquids (ILs), alkali/urea aqueous systems and metal salts (lithium chloride, LiCl)/N,N-dimethylacetamide (DMAc) solutions dissolve cellulose under different conditions [7,8]. However, most methods used to dissolve cellulose need rigorous conditions, such as long-term pretreatment, high temperature of 150° C., and high recycling cost [9-14].
Traditional conductive materials, such as noble metals like copper (Cu), silver (Ag) and gold (Au), are the major components used in energy, consumer electronics and semiconductor industry. They generally exhibit the best conductive properties. However, due to the high recycling cost and some defects in apparent mechanical properties such as folding or bending, new emerging materials are gradually showing better advantages. For example, carbon nanotubes (CNTs) and graphene oxide (GO) are two important conductive materials that have been used as fillers to reinforce polymer matrixes and enhance mechanical strength and electrical conductivity [15,16]. Recent studies reported on cellulose based composite materials using CNT or GO as fillers to obtain superior conductivity of the raw cellulose, which opens the possibility to extend the application of cellulose to high-tech electrical and energy industries [17-20]. Although the protocol for preparing these composites is quite easy (dissolving cellulose, mixing dispersed CNT or GO particles, and then perform film casting), making homogenous and well-designed products is difficult.
3D printing is a cutting-edge technology of great interests. It is capable of rapidly and accurately prototyping and patterning a material model in terms of various requirements [21]. To date, very few studies and almost no specific patents on pure cellulose composites with a fine structure prepared by 3D printing technology have been documented, especially for applications in electrical and energy industries. Markstedt et al. [22] reported a 3D-printing cellulosic products made of dissolved cellulose in an ionic liquid (IL) (1-ethyl-3-methylimidazolium acetate) and they proposed that the high concentration of cellulose in the solvent would assist in the printing process. Accordingly, 3D printing of dissolved cellulose combining compatible CNT or GO conductive ingredients to make a homogeneous conductive composite is feasible and deserves more studies.